Posterior Inferior Serratus Muscle

Description

Posterior thoracic wall anatomy is centered on the serratus posterior inferior, a thin quadrilateral muscle with serrated digitations that descend inferolaterally from the lower thoracic and upper lumbar spinous processes toward the posterior aspects of ribs 9 through 12. Its slips lie deep to latissimus dorsi and superficial to the erector spinae fascia, placing the muscle on the posterolateral thorax just inferior to the inferior angle of the scapula. Medially, the fibers approach the vertebral column; laterally, the digitations spread over the lower ribs where the “jagged” border is most apparent. Teaching value comes from its position at the junction of back musculature and the mechanics of respiration. Although often described as a rib depressor, serratus posterior inferior is frequently treated clinically as a proprioceptive and postural muscle, and its fascial connections to thoracolumbar fascia can contribute to lower thoracic myofascial pain patterns that mimic costovertebral or flank discomfort. Clear separation from latissimus dorsi also matters during posterior approaches for thoracolumbar exposure, where respecting tissue planes reduces bleeding and preserves the integrity of the thoracolumbar fascia. Small muscle, common source of confusion. Use this illustration in back and thorax lab manuals to reinforce layered dissection order (latissimus dorsi, thoracolumbar fascia and erector spinae, then serratus posterior inferior), and in respiratory mechanics lectures to anchor discussion of rib 9 to 12 motion and costovertebral joint landmarks. It also fits well in surgical anatomy references that describe posterior incisions and the muscle planes encountered over the lower ribs in adult male anatomy. Anatomical accuracy verified by SciePro's Medical Advisory Board.